A steel material used in a chemical plant is required to have excellent corrosion resistance as well as strength. In particular, in a urea plant that is one of chemical plants, high temperature strength and nitric acid corrosion resistance are required. In a urea plant, urea is generally produced by the following method. A gaseous mixture containing ammonia and carbon dioxide is condensed through a high pressure of 130 kg/cm2 or higher in a high temperature range of 160 to 230° C. At this time, urea is produced by synthesis reaction. Since urea is produced under a high temperature and high pressure as described above, the steel materials used in urea plants are required to have excellent high temperature strength.
In the production process of urea described above, an intermediate product called ammonium carbamate is further produced. Corrosiveness of ammonium carbamate is very strong. It is generally known that corrosion by ammonium carbamate is correlated with corrosion by nitric acid. Accordingly, steel materials for urea plants are required to have not only high temperature strength but also excellent nitric acid corrosion resistance.
Austenitic stainless steel typified by SUS316, SUS317 and the like in JIS Standard has excellent corrosion resistance. Therefore, these types of austenitic stainless steel are used as steel materials for plants.
With the objective of further improving the strength and corrosion resistance of the austenitic stainless steel as above, the following arts are proposed.
JP10-88289A (Patent Document 1) proposes Cr—Mn austenitic steel excellent in strength and corrosion resistance. In Patent Document 1, the crystal grains of Cr—Mn austenitic steel are ultra-refined, and the average grain size is 1 μm or less. Patent Document 1 indicates that thereby, Cr—Mn austenitic steel having high strength and excellent corrosion resistance is obtained.
JP6-256911A (Patent Document 2) proposes austenitic stainless steel having excellent nitric acid corrosion resistance even after cold working. In Patent Document 2, Ni, Mn, C, N, Si and Cr contents in the steel are controlled. Patent Document 2 indicates that thereby, martensite production by strain induced transformation after cold working is suppressed, and excellent nitric acid corrosion resistance is obtained.
JP2005-509751A (Patent Document 3) proposes ultra austenitic stainless steel having excellent corrosion resistance. In Patent Document 3, Cu is contained as well as Cr, Ni, Mo and Mn. Patent Document 3 indicates that by containing right amounts of these elements, excellent corrosion resistance is obtained.
However, the austenitic stainless steel disclosed in each of Patent Documents 1 to 3 sometimes cannot provide sufficient high temperature strength while maintaining nitric acid corrosion resistance.